Optical communication requires optical components which distribute the power of individual channels from n incoming channels over n outgoing channels. When the number of channels exceeds 2, these components are designated as star couplers. These couplers should have the lowest possible insertion losses and distribute the power as uniformly as possible over the n outgoing channels, independent of the excitation mode.
DE-OS No. 30 which corresponds substantially to U.S. Pat. No. 4,484,794, 36 044 discloses an optical star coupler in which two optical fibre bundles are interconnected via a mixer element designed as a planar waveguide. The mixer element has a thickness which is substantially equal to the fibre diameter. The optical fibres incoming at its output and outgoing from its output are located in the same plane and are coupled such that their packing density is as great as possible. The diameter of the optical fibres is substantially equal to the core diameter of the system fibres to be connected to the star coupler. The mixer element operates together with the incoming optical fibres coupled thereto as an optical multiplexer or an optical coupler, while the mixer element together with the outgoing optical fibres operates as an optical branching device.
An optical branching device is disclosed in DE-OS No. 23 40 020. This branching device is in the form of a three-port and can be produced in planar technique for both multimode and singlemode fibres.
Integrated single-mode opical 1.times.N star couplers are extensively described in Appl. Phys. Lett. 40 (7) dated Apr. 1st, 1982 on Pages 549 to 550. One of the configurations illustrated in FIG. 1 on Page 549 is implemented from cascaded three-ports. A light-conducting center channel is provided at which the three-ports are arranged sequentially. At each three-port each time one branching circuit is inclined from each side under the same angle to the center channel. In the embodiment shown the branching circuits are not connected to the center channel. There is, to the contrary, a defined distance between them. The influences of the value of this distance and also the angle at which the branching circuits lead into the center channel are indicated. Because of the narrow branch angle the arrangement must be of a great length.
DE-OS No. 32 30 657 which corresponds substantially to U.S. Pat. No. 4,535,440, discloses an optical multiplexer constituted by cascaded three-ports which can also be utilized as a branching device. A light-conducting center channel is provided at which the three-ports are arranged sequentially and at each three-port a branching cirucit leads from each side at the same angle into the center channel. A branch angle of approximately 1.5.degree. is mentioned as being particularly advanatageous. The insertion losses at the three-ports are low and always the same. The light-conducting circuits are produced in a solid body made of, for example, boron oxide silicate glass in that ions which change the refractive index are introduced along desired paths, using a mask metal. A different method of producing these light-guiding circuits is disclosed in, for example, the report of the "First European Conference on Integrated Optics" London, 14/15 September 1981, by D. Hoffmann and U. Langmann, Pages 1-3. A LiNbO.sub.3 solid body is used as the substrate. The paths are produced by indiffusion of titanium ions.
In the article "Low-loss multiple-branching circuit Ti-indiffused LiNbo.sub.3 channel waveguides" in Optics Letters, Volume 9, no. 6, dated June 1984, Pages 246 to 248, the properties of a three-port manufactured using integrated circuit techniques are investigated. This article describes the analysis of the branch angle dependence of the power distribution between the middle branch to the sum of the outer branches wherein the branch angles investigated have values between 0.2.degree. and 2.5.degree..